A simple example of how to add a basic sidebar to a WordPress theme

functions.php

/**
 * Registers a new sidebar with the theme.
 * 
 * Note: replace 'themename' with the name of your theme.
 */
function themename_add_sidebar() {

	/*
	 * Description of the parameter array is as follows:
	 * 
	 * name:		The text that appears in the widgetized area in the Dahsboard
	 * id:			The ID of the sidebar. It's a good practice to use sidebar-0, sidebar-1, etc to maintain compatibility so that widgets maintain their position across themes.
	 * description:		The text that appears on the inside of the widgetized area in the Dashboard
	 * before_widget:	The markup written to the front end that starts the sidebar.
	 * after_widget:	The markup written to the front end that ends the sidebar.
	 * before_title:	The markup to display before the widget's title text.
	 * after_title:		The markup to display after the widget's title text.
	 *
	 */

	register_sidebar(
		array(
			'name' 		=> 'Sidebar Title',
			'id' 		=> 'sidebar-0',
			'description'	=> 'The primary sidebar',
			'before_widget' => '<div id="%1$s" class="widget %2$s">',
			'after_widget'  => '</div>',
			'before_title'  => '<h3 class="widget-title">',
			'after_title'   => '</h3>'
		)
	);

}
add_action( 'widgets_init', 'themename_add_sidebar' );

sidebar.php

<?php // If there's a widget in the 'Sidebar, display it ?>
<?php if ( is_active_sidebar( 'sidebar-0' ) ) {  ?> 
 
	<div id="widget">
		<?php dynamic_sidebar( 'sidebar-0' ); ?>
	</div><!-- /#header-widget -->							

<?php // Otherwise, take some other action like writing out default text or markup ?>
<?php } else { ?>
	<!-- Write out whatever markup you want. -->
<?php } // end if ?>